Investigations of the major histocompatibility complex of the rabbit (RLA) have led to identification of a nonpolymorphic gene encoding the class II DO beta chain. DOB has an overall structural similarity to other class II beta chains but unlike these, it is expressed only on mature B cells and to a limited extent in thymus. The function of the DOB chain is of interest so the gene is being expressed in an E. coli protein system in order to prepare monoclonal antibodies against it. The gene encoding the putative alpha chain partner of DOB was sought using probes for the human counterpart and candidate clones are being studied. Efforts to construct a complete physical map of the rabbit MHC are in progress and attempts to order the various MHC regions in the rabbit continue. Previous results showed that class II and class I regions were adjacent with no interposition of Class III genes. Studies of class III gene prompted a study of heat shock protein 70 which is expressed on HTLV-I infected cells. Anti-heat shock protein (hsp) antibodies were observed in the sera of rabbits after inoculation with the HTLV-I cell lines. One cell line RH/K34, which failed to raise a response to hsp70, was shown subsequently to cause lethal leukemia when live cells were injected into unrelated outbred rabbits. Injection of sublethal doses of this line failed to elicit anti hsp although injection of cell free virus from the line did so. Rabbits with detectable levels of anti hsp70 antibodies prior to challenge with lethal doses of live RH/K34 cells were resistant to its lethal effects suggesting that hsp immunity (which is autoimmunity) may influence the outcome of RH/K34 pathogenicity and may have general implications to viral immunity.